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Enhanced production of methane from waste activated sludge by pretreatment using a gas-diffusion cathode Yufeng Jia, Huajun Feng, Dongsheng Shen, Yuyang Zhou, Yanfeng Wang, Wei Chen, and Bin Huang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02151 • Publication Date (Web): 18 Nov 2016 Downloaded from http://pubs.acs.org on November 21, 2016

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Table Caption Table 1. Characteristics of The WAS Used in The Present Study.

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Table 1. Characteristics of The WAS Used in The Present Study. Items

Average (±standard deviation)

pH

6.98 ±0.03

Soluble chemical oxygen demand （SCOD, mg/L）

105 ±13

Total chemical oxygen demand 7855 ±183 (TCOD, mg/L) Soluble carbohydrate (mg/L)

19 ±3

Soluble protein (mg/L)

26 ±4


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Figure Caption Figure 1. H2O2 Yields from The Surfaces of Gas-diffusion Cathodes Following Electrochemical Deposition with Varying Cycles, as Functions of The Applied Potential (vs Ag/AgCl). Figure 2. (a) SCOD, (b) Protein, (c) Carbohydrate Concentrations During Pretreatment and (d) The Effect of Pretreatment on The Disintegration of The WAS. Figure 3. The Cumulative Methane Production During The Digestion. Figure 4. Performance of The GDC During Long Term Operation for 48 h at – 1.0 V. Figure 5. SCOD Concentrations under Different Conditions. 0: Control; A, H2O2; B, pH; C, pH +H2O2; 1, − 0.8 V; 2, −1.0 V; 3, −1.2 V.


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Figure 1. H2O2 Yields from The Surfaces of Gas-diffusion Cathodes Following Electrochemical Deposition with Varying Cycles, as Functions of The Applied Potential (vs Ag/AgCl).


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Figure 2. (a) SCOD, (b) Protein, (c) Carbohydrate Concentrations During Pretreatment and (d) The Effect of Pretreatment on The Disintegration of The WAS.


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Figure 3. The Cumulative Methane Production During The Digestion.


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Figure 4. Performance of The GDC During Long Term Operation for 48 h at – 1.0 V.


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Figure 5. SCOD Concentrations under Different Conditions. 0: Control; A, H2O2; B, pH; C, pH +H2O2; 1, − 0.8 V; 2, −1.0 V; 3, −1.2 V.


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Enhanced production of methane from waste activated sludge by pretreatment using a gas-diffusion cathode

Yufeng Jiaa,b, Huajun Fenga,b*, Dongsheng Shena,b, Yuyang Zhoua,b, Yanfeng Wanga,b, Wei Chena,b, Bin Huan a,b

a

School of Environmental Science and Engineering, Zhejiang Gongshang University,

Hangzhou310012, P.R. China

b

Zhejiang Provincial Key Laboratory of Solid Waste Treatment and Recycling,

Hangzhou 310012, P.R. China

*E-mail: [email protected] Address: School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou 310012, China. Tel.: +86 571 87397126; fax: +86 571 87397126.

1


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ABSTRACT: Anaerobic digestion is an alternative technology for the pretreatment of waste activated sludge (WAS) before final disposal. Hydrolysis is the rate limiting step in this process, so pretreatment of WAS to allow for high-efficiency anaerobic digestion is beneficial. In the present work, an electrochemical system characterized by a gas-diffusion cathode (GDC) was established to facilitate WAS pretreatment. This GDC was composed of a carbon black diffusion layer and a Ni/NiOx catalytic layer. H2O2, an essential component of the pretreatment, was evolved on the electrode surface at a rate of 27.3 mg/cm2d. On varying the potential of the GDC from 0 to −1.0 V (vs Ag/AgCl), the soluble chemical oxygen demand (SCOD) , the protein and carbohydrate concentrations increased from 134 ± 6, 38 ± 6 and 20 ± 3 mg/L to 3617 ± 65, 1436 ± 121 and 470 ± 64 mg/L, respectively, over 8 h. The ratio of SCOD to the total chemical oxygen demand (TCOD) was 48% ± 1.3%. The total methane output of the GDC-pretreated sludge at the end of 10 d was close to 234 mL/g TCOD, which was 33.8% greater than that of the control sludge. The Ni/NiOx-modified GDC also exhibited excellent stability performance. The results of this work demonstrate the pivotal role of the combination of H2O2 and alkali generated by the GDC in the enhanced pretreatment of WAS.

KEYWORDS: gas-diffusion cathode; Ni/NiOx-modification; pretreatment; waste activated sludge

2


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1. INTRODUCTION It has been determined that, each year, over 34 million tons of waste activated sludge (WAS), the primary byproduct of the activated sludge process containing complex organic compounds, viruses, bacteria and other microorganisms

1,2,3

, is

improperly disposed of in China. Conventional WAS treatment methods are costly 3, and include landfill, composting, and incineration

4,5,6

, all of which can easily lead to

secondary environment contamination, such as the accumulation of undesirable substances (including heavy metals, pathogens and organic pollutants) 7 or the release of harmful gases (such as dioxins, furans, NOx, N2O and CO) 8. Recently, anaerobic digestion has received increasing attention as a means of treating WAS so as to ensure its stabilization by reducing volatile compounds and extracting methane 9. However, this method is currently limited by the associated hydrolysis process, which is a crucial step in anaerobic digestion10. Therefore, pretreating WAS to improve its degradability would enhance the production of methane during sludge treatment and is of significant interest. The main goal of the anaerobic digestion of WAS is to remove organic material in the sludge, and various pretreatment techniques have been widely applied for this purpose, including mechanical, thermal, chemical and biological methods

11,12,13,14

.

One so-called downstream approach, alkali treatment, involves adding chemical reagents (such as NaOH) and is the most common method at present owing to its effectiveness. Rajanet al.15 chemically pretreated WAS with NaOH and reported an increase in solubilization of more than 46%. Advanced oxidation using oxidative 3


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reagents is another alternative for efficient treatment. Kim et al.

16

studied the

pretreatment of WAS with H2O2 (1.6 M) and found that the soluble chemical oxygen demand (SCOD) to total chemical oxygen demand (TCOD) ratio increased from 0.02 (in the raw sludge) to 0.55. Despite this high efficiency, these alkaline and oxidation processes have several associated drawbacks. The addition of alkaline reagents tends to increase the salinity of the sludge, which is detrimental to the subsequent biological digestion

steps17,18.

For

these

reasons,

the

exploration

of

efficient

and

environment-friendly technologies for sludge pretreatment is necessary. The air cathode (Figure S1)19, a newly-emerging and green technology, could utilize electron to synthesize hydrogen peroxide (H2O2) 20. Rozendal et al.21 reported that H2O2 can be efficiently generated using air-cathodes composed of a graphite rod and a bed of graphite granules, with a production rate 1.9 ± 0.2 kg/ m3 d. This type of cathode can also reductively degrade various pollutants in aqueous solution

22,23

.

During this process, cathodic proton consumption generates an alkaline environment as the result of hydroxyl ion accumulation. It has been reported that an applied voltage of 1.0 V will produce a pH value in the catholyte of 12 after 18 h of operation 24

. Thus, simultaneous advanced oxidation and alkalization can be achieved in an air

cathode-based electrochemical system. In the present study, a gas-diffusion cathode (GDC) modified with Ni/NiOx was employed to improve the H2O2 generation rate. This type of cathode, in which alkali and H2O2 are generated simultaneously, has been proposed as an efficient and green method of WAS pretreatment. The objective of this study was to investigate the 4


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feasibility of WAS pretreatment using this device. In addition, the mechanism of WAS treatment in conjunction with this type of cathode is discussed. 2. EXPERIMENTAL SECTION 2.1. WAS. The WAS used in these trials was obtained from a municipal wastewater treatment plant in Hangzhou, China, and was generated from an activated sludge treatment process that purifies 1.2 × 106 tons of wastewater daily. The collected sludge samples were first screened and filtered using a 40-mesh sieve to remove large particles, then washed three times with deionized water and stored at 4 °C until use. The characteristics of the WAS are provided in Table 1. 2.2. Fabrication of The GDC. The GDC was prepared according to the diagram in Figure S2. Briefly, carbon black (40 nm, EC-600JD, Lion Corporation, Japan) and graphite (100 µm, >99.95%, Xinyuan Chemical Reagent Co., Ltd., Henan, China) were cleaned by ultrasonication in deionized water for 60 min at 25°C. After that, 1 g carbon black and 4 g graphite were dispersed in 50 mL ethanol in a conical flask held in an ultrasonic bath for 25 min. A polytetrafluoroethylene (10.5 g) emulsion (60%; Dupan, USA), was gradually added to the mixture to produce the CL as previously described25. The mixture was subsequently stirred at approximately 85 °C to obtain a dough-like paste, which was rolled to form a 0.4-mm-thick CL film and then roll-pressed onto one side of a stainless steel mesh (4.0 × 4.0 cm, 316L, 50-mesh, Xinan Commercial Trade Co., Ltd., Shanghai, China), resulting in a flat sheet. The GDLs were rolled onto the opposite side of the CLs to form the final GDCs, each with a total thickness of 0.8 mm, followed by heating at 340 °C for 30 min. 5


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Ni/NiOx films were synthesized on the electrode surfaces by electrochemical deposition. The electrochemical deposition occurred over the range of 0 to −1.5 V at a scan rate of 20 mV/s in a solution of 10 g/L Ni2SO4 in water. Different cycle times were found to have a significant effect on the amount Ni/NiOx deposited on the CL. In this work, the amount of Ni/NiOx deposited on the CL was therefore determined by adjusting the cycle time during electrochemical deposition, using values of 0, 5, 10, and 15. Subsequent to deposition, the GDC was washed in distilled water and then dried at ambient temperature. 2.3. Reactor Construction and Operation. Each experimental trial was performed in a three-chamber reactor with the construction shown in Figure S2. The chambers were 4.0 cm in length and 2.0 cm in radius, with a total volume of 160 mL. The anode and cathode chambers were assembled on either side of a cation exchange membrane (NR211, Dupan, USA) and held securely in place. The anode was made of carbon brushes (4.5 cm in length and 2.0 cm in radius). During the various trials, the GDC was raised to a poised potential (either 0, −0.8, −1.0 or −1.2 V vs. Ag/AgCl) and the H2O2 concentration was monitored at 20 min intervals. A Ag/AgCl electrode was used as the reference electrode, and the GDC was fed with air or N2 at a rate of 300 mL/min. All potentials in this work were quoted relative to the Ag/AgCl reference electrode. 2.4. Methane Production Yields. Methane yields were assessed under various conditions using a series of 120 mL serum bottles, each with a 100 mL working volume, at 37 °C. The bottles were filled with 30 mL of inoculum and 70 mL of a 6


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sludge sample. Before the start-up of the reactors, the pH of each digester was carefully checked and the solution was neutralized so as to obtain a pH value of approximately 7.0. Once the reactors were loaded, the headspace of each was purged with high-purity N2 at a flow rate of 300 mL/min for 20 min to remove atmospheric oxygen. The volumes of methane produced during these tests were measured periodically by releasing the pressure in the bottles. The biogas volume produced by anaerobic digestion was measured by using water displacement through a fermentation tube. The methane content in biogas was determined by chromatography. For this analysis, nitrogen was used as carrier gas. The detector was a Thermal Conductivity Detector (TCD) and a Hayesep R packed GC column (3 m × 2 mm). Gas was collected using a Gastight Hamilton 1 mL syringe. The temperatures of the injector and the detector were both 100 °C. 2.5. Chemical and Electrochemical Analysis. During these trials, 2 mL sludge aliquots were taken from the reactors at 2 h intervals for analysis. These samples were centrifuged at 10,000 rpm for 10 min and immediately filtered through 0.45 µm cellulose membrane filters in preparation for the subsequent analysis of SCOD, soluble protein, and soluble carbohydrates. TCOD and SCOD were determined according to the procedures provided by the Standard Methods for the Examination of Water and Wastewater

26

. Protein concentrations were determined with Lowry’s

method, using bovine serum albumin as a standard solution

27

. Carbohydrates were

measured with the phenol sulfuric acid method, using a standard glucose solution 28. Scanning electron microscopy (SEM; Biologic VSP, Claix, France) images of the 7


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electrode surfaces were obtained at an acceleration voltage of 5 kV. Cyclic voltammetry was carried out within the potential window of −1.5 to 0 V at a scan rate of 20 mV/s in M9 solution. This solution consisted of 0.1 g/L NH4Cl, 0.5 g/L NaCl, 4.4 g/L KH2PO4, 3.4 g/L K2HPO4, 0.1 g/L MgSO4, 2 g/L NaHCO3, and 1.0 mg/L FeSO4·7H2O 29. 3. RESULTS AND DISCUSSION 3.1. H2O2 Production from GDCs. Figure 1 showed the effect of varying the number of electrochemical deposition cycles on H2O2 production. As the cycle number was increased, the H2O2 generation rate also increased significantly. The data show that a cycle value of 10 or 15 gave a H2O2 yield that was both optimized and stable, with a maximum production rate of 27.3 mg/cm2d at a potential of −1.0 V. Figure S4 presented SEM images of the Ni/NiOx film on the GDC surface as the number of cycle increased gradually from 0 to 15. Figure S5 showed a much higher cathodic current was generated in the Ni/NiOx-modified GDC than in the unmodified GDC, indicating higher electrochemical activity in the former. A previously reported system with air cathode achieved hydrogen peroxide production of 0.96 mg/cm2d, which was far from competing yield in our research (27.3 mg/cm2d), the gap of which derived from Ni/NiOx modification here21. The enhancement of cathodic current demonstrated that the presence of Ni/NiOx on the GDC surface improved the interfacial charge transfer and catalyzed the electrochemical reduction. From the above, we inferred that modification of the GDC by Ni/NiOx had a significant effect on the H2O2 production rate. 8


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It was also observed that, under all conditions, the H2O2 generation rate did not increase linearly with potential. As well, during electrolysis, the coulombic efficiency was found to slowly decrease. Based on a report by Boye et al. 30, these results can be largely ascribed to the self-decomposition of the H2O2 to produce O2 or its reduction to water at the GDC surface. The H2O2 production rate reached a maximum of 27.3 mg/cm2d, when the potential was maintained at −1.0 V. It is interesting to note that the H2O2 production rate did not vary with the number of cycles, on the contrary, slightly reduction. The SEM images in Figure S4d demonstrate that Ni/NiOx completely covered the electrode surface following 15 cycles. However, a lower rate of H2O2 generation was achieved on the excessively thick Ni/NiOx layer. Based on these results, a GDC fabricated with 10 cycles was employed for the remaining aspects of this study. 3.2. Pretreatment Performance. During the pretreatment process, the SCOD in the supernatant underwent an obvious increase as a function of time. During the initial pretreatment stage (the first 6 h), the SCOD increased dramatically from 102 ± 12 mg/L to 3050 ± 176 mg/L in accordance with the applied potential increases from 0 V to −1.2 V, respectively. Upon prolonged pretreatment (from 6 to 8 h), the SCOD concentration in the liquid phase rose more gradually. The SCOD values in the solution after 8 h were 3548 ± 145, 3617 ± 65 and 3768 ± 79 mg/L for −0.8, −1.0 and −1.2 V, respectively. However, the concentration of SCOD was found to be 15% to 18% lower than that in a control experiment under N2 at the same potential. A number of studies with cathode chambers have demonstrated that H2O2 can be generated 9


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in-situ by the reduction of oxygen in an alkaline environment [Eq.(1)] in an electrochemical system 31. O2 + H2O + 2e- → HO2-+OH- (1) Considering the powerful oxidative activity of H2O2, it can be expected to damage the cellular structures within the sludge and thus to promote the dissolution of organic matter. At a cathodic potential of−0.8 V, 8 h was required for the dissolution of 3600 mg/L SCOD. Upon decreasing the applied potential to −1.2 V, this period was reduced to 4 h. Proteins and carbohydrates are the main degradable organics in WAS and also account for the bulk of the extracellular polymeric substances (EPS) in sludge. Therefore, the extraction process depends on the dissolution of these organic materials. Figure 2 depicted the variations in the protein (Figure 2b) and carbohydrate (Figure 2c) concentrations during the pretreatment process. Similarly, the concentration of proteins increased to 1436 mg/L after 8 h under the condition of supplying with air, which was 30.5% higher than those in the solutions supplied with N2, at an applied cathodic potential of −1.2V. The carbohydrate concentrations abruptly rose to 470 mg/L, which was in agreement with the variation of protein. The above results suggest that GDC pretreatment can efficiently degrade the bound EPS so as to release various organisms, and that the surrounding atmosphere has a significant effect on this process. The effects of pretreatment on sludge disintegration are summarized in Figure 2d. Here the SCOD level in the raw sludge is only 1.38% of the TCOD. After 8h of 10


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pretreatment with the GDC under various potentials, the SCOD increased to 46.5% to 48% of the TCOD. These results are comparable to previously published data showing that a solubilization rate of more than 45% of the particulate COD was achievable in 30 mmol/L NaOH32. However, there was no addition of chemical reagent in GDC pretreatment, the salinity concentration of electrolyte didn't change, which benefited subsequent anaerobic digestion. The above results demonstrate that GDC modified with Ni/NiOx has broad applications to the anaerobic digestion of WAS. Figure 3 showed that after 10 days of digestion, the accumulated methane production when using GDC to pretreat the sludge was close to 234 mL/g TCOD, or 33.8% higher than that of the control (155 mL/g TCOD). The higher yield obtained from the pretreated sludge samples can be explained by the fact that the pretreated sludge contained higher levels of soluble organic material, resulting in greater methane production. Hence, GDC pretreatment could be an effective means of enhancing the production of methane from WAS. The cumulative methane productions under various conditions were assessed to further evaluate the feasibility of using GDC pretreatment to improve the anaerobic biodegradability of WAS. 3.3. Stability of The GDC. The stability of a GDC modified with Ni/NiOx is one of the most important factors related to the eventual industrial application. In this work, the SCOD was taken as a useful parameter reflecting the stability of the GDC under long-term operation. After a GDC was used over a 48h period to process six batches, the SCOD concentration was only minimally changed and still reached 3519 11


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± 100 mg/L (p>0.05), a decrease of only about 3%, as shown in Figure 4. The Ni/NiOx-modified GDC therefore appears to be both stable and durable, and can be used for long-term operation in applications involving sludge pretreatment. 3.4. Analysis of The Pretreatment Mechanism. To determine the respective strength of H2O2 and alkali in WAS pretreatment, we carried out trials using concentrations of H2O2corresponding to the H2O2 yield of Figure 1. The pH of the WAS was also adjusted by adding 1 M NaOH so as to maintain the same pH as shown in Figure S6. Figure 5 demonstrated that adding an equal amount of H2O2 to treat the sludge generated a SCOD level of only about 500 mg/L. When alkali was used, however, the concentration of SCOD reached 2950 mg/L, equal to 85% of the SCOD level obtained from a combined pretreatment with H2O2 and alkali, which was similar to GDC based WAS pretreatment. Therefore, the effectiveness of the GDC-based WAS pretreatment was ascribed to the role of alkali. 4. CONCLUSIONS To the best of our knowledge, this work represented the first-ever pretreatment of WAS using a GDC modified with Ni/NiOx. The results showed that 10 cycles generate the maximum H2O2 yield of 27.3 mg/cm2d at a potential of −1.0 V, and that the combination of H2O2 and alkali produced in the GDC during treatment of the sludge has a satisfactory effect. The SCOD treated by GDC with a potential of −1.0 V was increased to 48% ± 1.3% of the TCOD. The time required for pretreatment was increased from 4 to 8 h upon changing the applied potential from −1.2 to −0.8 V. The accumulated methane production obtained from GDC pretreated sludge at the end of a 12


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10 d digestion period was nearly 234 mL/g TCOD, or 33.8% higher than the value obtained from the control. The generation of alkali was found to play the main role in the pretreatment. The Ni/NiOx-modified GDC exhibited high stability and, based on all the above results, this cathode should have significant applications involving sludge pretreatment. ACKNOWLEDGMENTS Jia, Y. acknowledges funding by Xinmiao Talent Project in Zhejiang Province (Project 2016R408054); Feng, H. acknowledges funding by the National Natural Science Foundation of China (Project 51478431); Shen, S. and Zhou, Y. acknowledge funding by Science and Technology Planning Project of Zhejiang Province (Project 2015C33025); Wang, Y.; Chen, W. and Huang, B. acknowledge funding by Xinmiao Talent Project in Zhejiang Province (Project 2016R408074). ASSOCIATED CONTENT Supporting Information The schematic of the gas-diffusion cathode (Figure S1), process for the fabrication of the gas-diffusion cathode (Figure S2), schematic of the three-chamber reactor (Figure S3), SEM images (Figure S4), cyclic voltammograms (Figure S5), and sludge pH values following treatment (Figure S6). The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding author *E-mail: [email protected]

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Address: School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou 310012, China. Tel.: +86 571 87397126; fax: +86 571 87397126. Notes The authors declare no competing financial interest.

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(16) Kim, T. H.; Lee, S. R.; Nam, Y. K.; Yang, J.; Park, C. Desalination. 2009, 246, 275-284. (17) Feijoo, G.; Soto, M.; Mendez, R.; Lema, J. M. Enzyme. Microb. Technol. 1995, 17, 180-188. (18) Panizza, M.; Cerisola, G. Electro Acta. 2008, 54, 876-878. (19) Dong, H.; Yu, H.; Gao, N.; Wang, X. J. Power. Sources. 2013, 232, 132-138. (20) Kadier, A.; Kalil, M. S.; Abdeshahian, P.; Mohamed, A.; Azman, N. F.; Logroño, W.; Simayi, Y.; Hamid, A. A. Renew. Sust. Energy. Rev. 2016, 61, 501-525. (21) Rozendal, R. A.; Leone, E.; Keller, J.; Rabaey, K. Electrochem. Commun. 2009, 11, 1752-1755. (22) Aulenta, F.; Tocca, L.; Verdini, R.; Reale, P.; Majone, M. Environ. Sci. Technol. 2011, 44, 8444-8451. (23) Lohner, S. T.; Becker, D.; Mangold, K. M.; Tiehm, A. Environ. Sci. Technol. 2011, 45, 6491-6497. (24) Chen, S.; Liu, G.; Zhang, R.; Qin, B.; Luo, Y. Environ. Sci. Technol. 2012, 46, 2467-2472. (25) Dong, H.; Yu, H.; Wang, X.; Zhou, Q.; Feng, J. Water. Res. 2012, 46, 5777-5787. (26) APHA. Standard Methods for the Examination of Water and Wastewater, 20thed. American Public Health Association/American Water Works Association/ Water Environment Federation, Washington, DC, USA.1998. (27) Fr, B.; Griebe, T.; Nielsen, P. Appl. Microbiol. Biotechnol. 1995, 43, 755-761. 16


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Table Caption Table 1. Characteristics of The WAS Used in The Present Study.

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Table 1. Characteristics of The WAS Used in The Present Study. Items

Average (±standard deviation)

pH

6.98 ±0.03

Soluble chemical oxygen demand 105 ±13 （SCOD, mg/L） Total chemical oxygen demand 7855 ±183 (TCOD, mg/L) Soluble carbohydrate (mg/L)

19 ±3

Soluble protein (mg/L)

26 ±4

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Figure Caption Figure 1. H2O2 Yields from The Surfaces of Gas-diffusion Cathodes Following Electrochemical Deposition with Varying Cycles, as Functions of The Applied Potential (vs Ag/AgCl). Figure 2. (a) SCOD, (b) Protein, (c) Carbohydrate Concentrations During Pretreatment and (d) The Effect of Pretreatment on The Disintegration of The WAS. Figure 3. The Cumulative Methane Production During The Digestion. Figure 4. Performance of The GDC During Long Term Operation for 48 h at – 1.0 V. Figure 5. SCOD Concentrations under Different Conditions. 0: Control; A, H2O2; B, pH; C, pH +H2O2; 1, − 0.8 V; 2, −1.0 V; 3, −1.2 V.

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Figure 3. The Cumulative Methane Production During The Digestion.

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Figure 4. Performance of The GDC During Long Term Operation for 48 h at – 1.0 V.

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Figure 5. SCOD Concentrations under Different Conditions. 0: Control; A, H2O2; B, pH; C, pH +H2O2; 1, − 0.8 V; 2, −1.0 V; 3, −1.2 V.

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Enhanced Production of Methane from Waste ... - ACS Publications

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